Method of interconnecting nanowires and transparent conductive electrode

ABSTRACT

According to embodiments of the present invention, a method of interconnecting nanowires is provided. The method includes providing a plurality of nanowires, providing a plurality of nanoparticles, and fusing the plurality of nanoparticles to the plurality of nanowires to interconnect the plurality of nanowires to each other via the plurality of nanoparticles. According to further embodiments of the present invention, a nanowire network and a transparent conductive electrode are also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority of U.S. provisionalapplication No. 62/109,776, filed 30 Jan. 2015, the content of it beinghereby incorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

Various embodiments relate to a method of interconnecting nanowires, ananowire network and a transparent conductive electrode.

BACKGROUND

Indium tin oxide (ITO) has been the most commonly used material as atransparent electrode for flat panel displays. However, it has somedrawbacks: it cannot be used for flexible displays which is the nextgeneration of display due to its brittle nature, as well as thedwindling supply of indium.

Recently, nanomaterials such as metallic silver (Ag) nanowires, carbonnanotubes (CNT) and graphene have been investigated as potentialreplacement materials for transparent electrodes on flexible substratesinstead of ITO. A number of studies have been carried out using anetwork of Ag nanowires as an electrode material. However, the cost ofAg itself is high and is comparable with the cost of ITO. On the otherhand, copper (Cu) is an abundant and cheap material, and is the dominantmetal used as an electrical conductor. It has a high electricalconductivity and is used as an electrode in conventional electronics.Though it has many advantages, Cu cannot be used as a transparentelectrode in its current form due to some of the reasons as listedbelow.

-   -   It is difficult to use Cu with other materials because Cu        usually reacts with other metals very aggressively to form        intermetallic compounds which degrade its properties.    -   Cu oxidizes in ambient condition, and more severely at higher        temperatures. The melting temperature of Cu (1084° C.) is much        higher than the processing temperature (<250° C.) of transparent        electrodes on typical flexible substrates, e.g., Polyimide (PI,        T_(g):340° C.), Polycarbonate (PC, T_(g):156° C.),        Polyethersulfone (PES, T_(g):223° C.), Polyethyleneterephthalate        (PET, T_(g):78° C.), Polyethylenenaphthalate (PEN, T_(g):121°        C.), Polyarylate (PAR, T_(g):350° C.).

SUMMARY

According to an embodiment, a method of interconnecting nanowires isprovided. The method may include providing a plurality of nanowires,providing a plurality of nanoparticles, and fusing the plurality ofnanoparticles to the plurality of nanowires to interconnect theplurality of nanowires to each other via the plurality of nanoparticles.

According to an embodiment, a nanowire network is provided. The nanowirenetwork may include a plurality of nanowires interconnected to eachother via a plurality of nanoparticles fused to the plurality ofnanowires.

According to an embodiment, a transparent conductive electrode isprovided. The transparent conductive electrode may include a nanowirenetwork, the nanowire network including a plurality of conductivenanowires interconnected to each other via a plurality of conductivenanoparticles fused to the plurality of conductive nanowires.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to like partsthroughout the different views. The drawings are not necessarily toscale, emphasis instead generally being placed upon illustrating theprinciples of the invention. In the following description, variousembodiments of the invention are described with reference to thefollowing drawings, in which:

FIG. 1A shows a flow chart illustrating a method of interconnectingnanowires, according to various embodiments.

FIG. 1B shows a schematic top view of a nanowire network, according tovarious embodiments.

FIG. 1C shows a schematic top view of a transparent conductiveelectrode, according to various embodiments.

FIG. 2A shows, as cross-sectional views, various processing stages of amethod of manufacturing copper (Cu) nanowires, according to variousembodiments.

FIG. 2B shows a method of mixing copper nanoparticles (Cu NPs) andcopper nanowires (Cu NWs), according to various embodiments.

FIGS. 2C and 2D show examples of some steps of the processing method ofvarious embodiments.

FIGS. 3A and 3B show scanning electron microscope (SEM) images of coppernanowires (Cu NWs) after manufacturing and after dispersion on asubstrate respectively.

FIG. 4 shows a schematic diagram illustrating the mechanism in whichcopper (Cu) nanoparticles fuse at the junctions of copper (Cu)nanowires.

FIG. 5A shows scanning electron microscope (SEM) images of coppernanowires (Cu NWs), according to various embodiments.

FIG. 5B shows scanning electron microscope (SEM) images ofmicrostructures of mixed copper (Cu) nanowires and copper (Cu)nanoparticles which are annealed at 200° C., according to variousembodiments.

FIG. 5C shows scanning electron microscope (SEM) images ofmicrostructures of mixed copper (Cu) nanowires and copper (Cu)nanoparticles, according to various embodiments.

FIG. 6 shows a plot of sheet resistance for various copper (Cu)nanostructures mixtures.

FIG. 7A shows a plot of sheet resistance of fused copper (Cu)nanowires-nanoparticles on different substrates.

FIG. 7B shows a plot of optical transmittance of a transparent coppernanowires (Cu NWs) with copper nanoparticles (Cu NPs) electrode.

FIG. 8A shows a schematic diagram illustrating a solution ofpolymethylmethacrylate (PMMA) with copper (Cu) nanowires dispensed ontoa substrate, according to various embodiments.

FIG. 8B shows scanning electron microscope (SEM) images illustrating theeffects of different concentrations of polymethylmethacrylate (PMMA) fordispersion.

FIG. 8C shows scanning electron microscope (SEM) images of copper (Cu)nanowires in polymethylmethacrylate (PMMA).

FIG. 9A shows schematic diagrams illustrating the use of an anodicaluminium oxide as a substrate in the method of various embodiments.

FIG. 9B shows photographs illustrating the transfer of nanowires from ananodic aluminium oxide to another substrate, according to variousembodiments.

FIG. 9C shows a scanning electron microscope (SEM) image of nanowires onan anodic aluminium oxide (AAO) substrate.

FIG. 10 shows a schematic diagram illustrating thiol molecules attachedon the surface of nanowires (NWs).

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawingsthat show, by way of illustration, specific details and embodiments inwhich the invention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention. Other embodiments may be utilized and structural, logical,and electrical changes may be made without departing from the scope ofthe invention. The various embodiments are not necessarily mutuallyexclusive, as some embodiments can be combined with one or more otherembodiments to form new embodiments.

Embodiments described in the context of one of the methods or devicesare analogously valid for the other methods or devices. Similarly,embodiments described in the context of a method are analogously validfor a device, and vice versa.

Features that are described in the context of an embodiment maycorrespondingly be applicable to the same or similar features in theother embodiments. Features that are described in the context of anembodiment may correspondingly be applicable to the other embodiments,even if not explicitly described in these other embodiments.Furthermore, additions and/or combinations and/or alternatives asdescribed for a feature in the context of an embodiment maycorrespondingly be applicable to the same or similar feature in theother embodiments.

In the context of various embodiments, the articles “a”, “an” and “the”as used with regard to a feature or element include a reference to oneor more of the features or elements.

In the context of various embodiments, the phrase “at leastsubstantially” may include “exactly” and a reasonable variance.

In the context of various embodiments, the term “about” as applied to anumeric value encompasses the exact value and a reasonable variance.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items.

As used herein, the phrase of the form of “at least one of A or B” mayinclude A or B or both A and B. Correspondingly, the phrase of the formof “at least one of A or B or C”, or including further listed items, mayinclude any and all combinations of one or more of the associated listeditems.

FIG. 1A shows a flow chart 100 illustrating a method of interconnectingnanowires, according to various embodiments.

At 102, a plurality of nanowires (NWs) are provided.

At 104, a plurality of nanoparticles (NPs) are provided.

At 106, the plurality of nanoparticles are fused to the plurality ofnanowires to interconnect the plurality of nanowires to each other viathe plurality of nanoparticles.

In various embodiments, the plurality of nanoparticles may be fused toand in between the plurality of nanowires to interconnect the pluralityof nanowires to each other via the plurality of nanoparticles, forexample, at the junctions between the plurality of nanowires. This maymean that nanoparticles may be present at a joint or junction betweentwo nanowires to fuse the two nanowires to each other. Therefore, twonanowires may be fused together with nanoparticles at a junction so thatthe two nanowires may be interconnected to each other via thenanoparticles at the junction.

In other words, the plurality of nanowires may be fused to each otherthrough the plurality of nanoparticles. This may mean that the pluralityof nanowires may be interconnected to each other by means of theplurality of nanoparticles, rather than direct nanowire-nanowire joints.

In various embodiments, the nanoparticles may also be fused to eachother.

In various embodiments, the plurality of nanowires and the plurality ofnanoparticles may be made of the same material (e.g., metal), which mayencourage reaction between the nanowires and the nanoparticles.

In various embodiments, at 106, in order to fuse the plurality ofnanoparticles to the plurality of nanowires, the plurality of nanowiresand the plurality of nanoparticles may be subjected to a heatingprocess. This may mean that a heat treatment may be carried out to fusethe plurality of nanoparticles to (and in between) the plurality ofnanowires.

The heating process may be carried out at a predetermined temperature ofabout 250° C. or less (i.e., ≤250° C.), for example, between about 100°C. and about 250° C., or between about 120° C. and about 250° C.Therefore, a low temperature processing method (<250° C.) may beprovided.

The heating process may be carried out for a predetermined duration ofbetween about 6 minutes and about 60 minutes, for example, between about6 minutes and about 40 minutes, between about 6 minutes and about 20minutes, between about 20 minutes and about 60 minutes, between about 40minutes and about 60 minutes, or between about 10 minutes and about 30minutes.

In various embodiments, a predetermined peak temperature of the heatingprocess may be between about 200° C. and about 250° C., for example,between about 200° C. and about 220° C., between about 220° C. and about250° C., or between about 230° C. and about 250° C., e.g., at about 250°C.

In various embodiments, the heating process at the predetermined peaktemperature may be carried out for a predetermined duration of betweenabout 90 seconds and about 30 minutes, for example, between about 90seconds and about 20 minutes, between about 90 seconds and about 10minutes, between about 10 minutes and about 30 minutes, or between about5 minutes and about 10 minutes, e.g., for about 90 seconds.

In various embodiments, the plurality of nanowires and the plurality ofnanoparticles may be mixed together prior to fusing the plurality ofnanoparticles to the plurality of nanowires.

In the context of various embodiments, each nanoparticle of theplurality of nanoparticles may have a size (or diameter) of betweenabout 5 nm and about 20 nm, for example, between about 5 nm and about 15nm, between about 5 nm and about 10 nm, between about 10 nm and about 20nm, or between about 8 nm and about 15 nm. The plurality ofnanoparticles may have the same (or uniform) size (or diameter).

By having smaller sized nanoparticles (e.g., 5-20 nm, or <10 nm), theprocessing temperature of the heating process for fusing the pluralityof nanoparticles to the plurality of nanowires may be lower (for example<250° C., e.g., between about 100° C. and about 250° C.) as compared tolarger sized nanoparticles. For example, for nanoparticles with 40-100nm diameter, the process temperature required may be in the range of300-350° C. Further, smaller sized nanoparticles (e.g., 5-20 nm, or <10nm) have been found to be preferentially deposited at junctions wherenanowires come together.

In the context of various embodiments, each nanowire of the plurality ofnanowires may have a length of between about 10 μm and about 50 μm, forexample, between about 10 μm and about 40 μm, between about 10 μm andabout 30 μm, between about 20 μm and about 30 μm, between about 20 μmand about 50 μm, between about 30 μm and about 50 μm, or between about25 μm and about 40 μm. The plurality of nanowires may have the same (oruniform) length.

In the context of various embodiments, each nanowire of the plurality ofnanowires may have a diameter of between about 20 nm and about 200 nm,for example, between about 20 nm and about 100 nm, between about 20 nmand about 50 nm, between about 50 nm and about 200 nm, between about 100nm and about 200 nm, between about 100 nm and about 150 nm, betweenabout 100 nm and about 120 nm, between about 150 nm and about 200 nm, orbetween about 120 nm and about 150 nm. The plurality of nanowires mayhave the same (or uniform) diameter.

In the context of various embodiments, each nanowire of the plurality ofnanowires may have an aspect ratio of between about 50 and about 500,for example, between about 50 and about 250, between about 50 and about100, between about 100 and about 500, or between about 100 and about300. The term “aspect ratio” may mean the length-to-width ratio orlength-to-diameter ratio.

In the context of various embodiments, a weight ratio of the pluralityof nanowires to the plurality of nanoparticles is between about 5:1 andabout 20:1, for example, between about 5:1 and about 10:1, between about10:1 and about 20:1, or between about 10:1 and about 15:1, e.g., about20:1. This may mean that the major/main constituent is the plurality ofnanowires while the minor constituent is the plurality of nanoparticles.It should be appreciated that having more of the plurality of nanowiresor the plurality of nanoparticles outside of the weight ratio asdescribed above may encourage agglomeration of the nanowires and/orincrease the processing temperature of the heating process for fusingthe plurality of nanoparticles to the plurality of nanowires.

In various embodiments, at 102, the plurality of nanowires may beprovided by forming the plurality of nanowires by means of anelectroplating method using an anodic aluminum oxide (AAO) as atemplate. The anodic aluminum oxide (AAO) may include holes or pores orchannels into which the material for the plurality of nanowires may beelectroplated to form the plurality of nanowires. The plurality ofnanowires may then be extracted or removed from the anodic aluminumoxide template. By employing an anodic aluminum oxide (AAO) (or anodizedaluminum oxide (AAO)) as a template, a uniform distribution of the size(e.g., length and/or diameter) of the plurality of nanowires may beobtained.

In various embodiments, at 102, the plurality of nanowires may bedispersed in a solvent to form a solution including the plurality ofnanowires, and, at 104, the plurality of nanoparticles may be added intothe solution. The solvent may act as a dispersing agent to disperse theplurality of nanowires so as to minimize agglomeration of the pluralityof nanowires. In this way, the solvent helps the plurality of nanowiresto have mobility and dispersibility. Further, the solvent may help tocarry, move or transfer the plurality of nanoparticles to thejunction(s) of the plurality of nanowires. The plurality ofnanoparticles may then be fused to the plurality of nanowires at suchjunction(s).

In various embodiments, at least some, or most, of the solvent may beremoved, evaporated or volatized during the heating process for fusingthe plurality of nanoparticles to the plurality of nanowires. Theplurality of (small and light) nanoparticles, or at least some of them,may then get together at the junctions between the plurality ofnanowires to find a position or spot where there may be lower surfaceenergy.

In various embodiments, the heating temperature of the heating processis higher than the boiling point of the solvent.

In the context of various embodiments, the solvent may be a lowviscosity liquid. As non-limiting examples, the solvent may include atleast one of ethanol, methanol, isopropyl alcohol (IPA), or ethyleneglycol.

In various embodiments, the method may further include depositing (ordispersing) the solution containing the plurality of nanowires and theplurality of nanoparticles on a substrate prior to fusing the pluralityof nanoparticles to the plurality of nanowires. This may mean thatfusing the plurality of nanoparticles to the plurality of nanowires, forexample, by means of a heating process, may be carried out after thesolution containing the plurality of nanowires and the plurality ofnanoparticles has been deposited on a substrate. In various embodiments,the heating temperature of the heating process is lower than thetransition temperature, Tg, of the substrate, but is higher than theboiling point of the solvent described above.

In various embodiments, the solution may be deposited on the substrateby at least one of spin coating, mayor bar coating, roll to roll coatingor spraying (spray coating).

In the context of various embodiments, the substrate may include aporous substrate, for example, anodic aluminium oxide (AAO).

In the context of various embodiments, the substrate may include aflexible substrate.

In the context of various embodiments, the substrate may include anorganic substrate.

In the context of various embodiments, the substrate may include atleast one of polyimide (PI), polycarbonate (PC), polyethersulfone (PES),polyethyleneterephthalate (PET), polyethylenenaphthalate (PEN) orpolyarylate (PAR).

In the context of various embodiments, the plurality of nanowires andthe plurality of nanoparticles may be conductive (e.g., electricallyconductive and/or thermally conductive).

In the context of various embodiments, the plurality of nanowires mayinclude a metal and/or the plurality of nanoparticles may include ametal. The metal may be selected from the group consisting of copper(Cu), silver (Ag) and gold (Au). In various embodiments, the pluralityof nanowires and the plurality of nanoparticles may include or may bemade of the same metal.

In the context of various embodiments, the plurality of nanowires andthe plurality of nanoparticles may include or consist essentially ofcopper (Cu).

In various embodiments, each nanowire of the plurality of nanowires mayinclude a surfactant on a surface of the nanowire. The surfactant mayprevent or minimize agglomeration of the plurality of nanowires. Thesurfactant may include a thiol (or thiol group) or an amine (or aminegroup). As non-limiting examples, the thiol group may includehexanethiol, octanethiol, decanethiol, dodecanethiol, etc. Asnon-limiting examples, the amine group may include hexylamine,octylamine, decylamine, dodecylamine, etc.

In other words, the plurality of nanowires may be treated to have asurfactant (e.g., thiol or amine) provided on the plurality ofnanowires. The treatment with the surfactant may be carried out beforemixing the plurality of nanowires with the plurality of nanoparticles.Generally, the process flow may be as follows: Nanowires (rawmaterial)→Treatment with surfactant (e.g., thiol or amine)→Addingnanoparticles→Mixing the nanowires and the nanoparticles with a matrix(e.g., polymethylmethacrylate (PMMA))→Dispersing (coating), for example,onto a substrate→Heating (annealing). The process of mixing with amatrix may be optional.

In various embodiments, each nanoparticle of the plurality ofnanoparticles may be encapsulated with an organic layer. This may meanthat each nanoparticle may be coated on its surface with an organiclayer. Therefore, the organic layer may be a capping layer. The organiclayer may prevent or minimize oxidation of the material of thenanoparticle. The organic layer may prevent or minimize agglomeration ofthe nanoparticles. The organic layer may be a polymeric layer. Invarious embodiments, the organic layer may be removed or volatizedduring the heating process for fusing the plurality of nanoparticles tothe plurality of nanowires.

It should be appreciated that, in general, the method may includemixing→dispersing→heating, where the plurality of nanowires and theplurality of nanoparticles may be mixed (e.g., in a solvent), and thendispersed (e.g., on a substrate), followed by heating to fuse theplurality of nanoparticles to (and in between) the plurality ofnanowires to interconnect the plurality of nanowires to each other viathe plurality of nanoparticles.

While the method described above is illustrated and described as aseries of steps or events, it will be appreciated that any ordering ofsuch steps or events are not to be interpreted in a limiting sense. Forexample, some steps may occur in different orders and/or concurrentlywith other steps or events apart from those illustrated and/or describedherein. In addition, not all illustrated steps may be required toimplement one or more aspects or embodiments described herein. Also, oneor more of the steps depicted herein may be carried out in one or moreseparate acts and/or phases.

FIG. 1B shows a schematic top view of a nanowire network 120, accordingto various embodiments. The nanowire network 120 includes a plurality ofnanowires 122 interconnected to each other via a plurality ofnanoparticles 124 fused to the plurality of nanowires 122. This may meanthat the nanowire network 120 may include interconnected nanowires 122.

In other words, a nanowire network (or network of interconnectednanowires) 120 may be provided. The nanowire network 120 may include aplurality of interconnected nanowires 122, and a plurality ofnanoparticles 124 fused to the plurality of interconnected nanowires 122such that the plurality of interconnected nanowires 122 areinterconnected to each other via the plurality of nanoparticles 124.

In various embodiments, the plurality of nanoparticles 124 may be fusedto and in between the plurality of nanowires 122 to interconnect theplurality of nanowires 122 to each other via the plurality ofnanoparticles 124 at the junctions 126 between the plurality ofnanowires 122. This may mean that nanoparticles 124 may be present at ajoint or junction 126 between two nanowires 122 to fuse the twonanowires 122 to each other. Therefore, two nanowires 122 may be fusedtogether with nanoparticles 124 at a junction 126 so that the twonanowires 122 may be interconnected to each other via the nanoparticles124 at the junction 126.

In other words, the plurality of nanowires 122 may be fused to eachother through the plurality of nanoparticles 124. This may mean that theplurality of nanowires 122 may be interconnected to each other by meansof the plurality of nanoparticles 124, rather than directnanowire-nanowire joints.

In various embodiments, the nanoparticles 124 may also be fused to eachother.

In various embodiments, the plurality of nanowires 122 and the pluralityof nanoparticles 124 may be made of the same material (e.g., metal),which may encourage reaction between the nanowires 122 and thenanoparticles 124.

In the context of various embodiments, individual (or individuallyresolvable) nanoparticles 124 of the plurality of nanoparticles 124 mayhave a size (or diameter) of between about 5 nm and about 20 nm, forexample, between about 5 nm and about 15 nm, between about 5 nm andabout 10 nm, between about 10 nm and about 20 nm, or between about 8 nmand about 15 nm. Individual (or individually resolvable) nanoparticles124 of the plurality of nanoparticles 124 may have the same (or uniform)size (or diameter).

In the context of various embodiments, individual (or individuallyresolvable) nanowires 122 of the plurality of nanowires 122 may have alength of between about 10 μm and about 50 μm, for example, betweenabout 10 μm and about 40 μm, between about 10 μm and about 30 μm,between about 20 μm and about 30 μm, between about 20 μm and about 50μm, between about 30 μm and about 50 μm, or between about 25 μm andabout 40 μm. Individual (or individually resolvable) nanowires 122 ofthe plurality of nanowires 122 may have the same (or uniform) length.

In the context of various embodiments, individual (or individuallyresolvable) nanowires 122 of the plurality of nanowires 122 may have adiameter of between about 20 nm and about 200 nm, for example, betweenabout 20 nm and about 100 nm, between about 20 nm and about 50 nm,between about 50 nm and about 200 nm, between about 100 nm and about 200nm, between about 100 nm and about 150 nm, between about 100 nm andabout 120 nm, between about 150 nm and about 200 nm, or between about120 nm and about 150 nm. Individual (or individually resolvable)nanowires 122 of the plurality of nanowires 122 may have the same (oruniform) diameter.

In the context of various embodiments, individual (or individuallyresolvable) nanowires 122 of the plurality of nanowires 122 may have anaspect ratio of between about 50 and about 500, for example, betweenabout 50 and about 250, between about 50 and about 100, between about100 and about 500, or between about 100 and about 300.

In various embodiments, the nanowire network 120 may be at leastsubstantially optically transparent. This may mean that the nanowirenetwork 120 may be at least substantially transparent to visible light.

In various embodiments, the nanowire network 120 may be conductive(e.g., electrically conductive and/or thermally conductive).

In the context of various embodiments, the plurality of nanowires 122may include a metal and/or the plurality of nanoparticles 124 mayinclude a metal. The metal may be selected from the group consisting ofcopper (Cu), silver (Ag) and gold (Au). In various embodiments, theplurality of nanowires 122 and the plurality of nanoparticles 124 mayinclude or may be made of the same metal.

In the context of various embodiments, the plurality of nanowires 122and the plurality of nanoparticles 124 may include or consistessentially of copper (Cu).

In various embodiments, each nanowire 122 of the plurality of nanowires122 may include a surfactant on a surface of the nanowire 122. Thesurfactant may prevent or minimize agglomeration of the plurality ofnanowires 122. The surfactant may include a thiol or an amine.

FIG. 1C shows a schematic top view of a transparent conductive electrode130, according to various embodiments. The transparent conductiveelectrode 130 includes a nanowire network 120 a, the nanowire network120 a including a plurality of conductive nanowires 122 a interconnectedto each other via a plurality of conductive nanoparticles 124 a fused tothe plurality of conductive nanowires 122 a. This may mean that thenanowire network 120 a may include interconnected conductive nanowires122 a.

In other words, a transparent conductive electrode 130 may be provided.The transparent conductive electrode 130 may have a conductive nanowirenetwork 120 a, which may include a plurality of interconnectedconductive nanowires 122 a, and a plurality of conductive nanoparticles124 a fused to the plurality of interconnected conductive nanowires 122a such that the plurality of interconnected conductive nanowires 122 aare interconnected to each other via the plurality of conductivenanoparticles 124 a.

In various embodiments, the plurality of conductive nanoparticles 124 amay be fused to and in between the plurality of conductive nanowires 122a to interconnect the plurality of conductive nanowires 122 a to eachother via the plurality of conductive nanoparticles 124 a at thejunctions 126 a between the plurality of conductive nanowires 122 a.This may mean that conductive nanoparticles 124 a may be present at ajoint or junction 126 a between two conductive nanowires 122 a to fusethe two conductive nanowires 122 a to each other. Therefore, twoconductive nanowires 122 a may be fused together with conductivenanoparticles 124 a at a junction 126 a so that the two conductivenanowires 122 a may be interconnected to each other via the conductivenanoparticles 124 a at the junction 126 a.

In other words, the plurality of conductive nanowires 122 a may be fusedto each other through the plurality of conductive nanoparticles 124 a.This may mean that the plurality of conductive nanowires 122 a may beinterconnected to each other by means of the plurality of conductivenanoparticles 124 a, rather than direct nanowire-nanowire joints.

In various embodiments, the conductive nanoparticles 124 a may also befused to each other.

In various embodiments, the plurality of conductive nanowires 122 a andthe plurality of conductive nanoparticles 124 a may be made of the samematerial (e.g., metal), which may encourage reaction between theconductive nanowires 122 a and the conductive nanoparticles 124 a.

In various embodiments, the nanowire network 120 a may be electricallyconductive and/or thermally conductive.

In various embodiments, the plurality of conductive nanowires 122 a andthe plurality of conductive nanoparticles 124 a may be electricallyconductive and/or thermally conductive.

In various embodiments, the nanowire network 120 a may be at leastsubstantially optically transparent. This may mean that the nanowirenetwork 120 a may be at least substantially transparent to visiblelight.

In the context of various embodiments, individual (or individuallyresolvable) conductive nanoparticles 124 a of the plurality ofconductive nanoparticles 124 a may have a size (or diameter) of betweenabout 5 nm and about 20 nm, for example, between about 5 nm and about 15nm, between about 5 nm and about 10 nm, between about 10 nm and about 20nm, or between about 8 nm and about 15 nm. Individual (or individuallyresolvable) conductive nanoparticles 124 a of the plurality ofconductive nanoparticles 124 a may have the same (or uniform) size (ordiameter).

In the context of various embodiments, individual (or individuallyresolvable) conductive nanowires 122 a of the plurality of conductivenanowires 122 a may have a length of between about 10 μm and about 50μm, for example, between about 10 μm and about 40 μm, between about 10μm and about 30 μm, between about 20 μm and about 30 μm, between about20 μm and about 50 μm, between about 30 μm and about 50 μm, or betweenabout 25 μm and about 40 μm. Individual (or individually resolvable)conductive nanowires 122 a of the plurality of conductive nanowires 122a may have the same (or uniform) length.

In the context of various embodiments, individual (or individuallyresolvable) conductive nanowires 122 a of the plurality of conductivenanowires 122 a may have a diameter of between about 20 nm and about 200nm, for example, between about 20 nm and about 100 nm, between about 20nm and about 50 nm, between about 50 nm and about 200 nm, between about100 nm and about 200 nm, between about 100 nm and about 150 nm, betweenabout 100 nm and about 120 nm, between about 150 nm and about 200 nm, orbetween about 120 nm and about 150 nm. Individual (or individuallyresolvable) conductive nanowires 122 a of the plurality of conductivenanowires 122 a may have the same (or uniform) diameter.

In the context of various embodiments, individual (or individuallyresolvable) conductive nanowires 122 a of the plurality of conductivenanowires 122 a may have an aspect ratio of between about 50 and about500, for example, between about 50 and about 250, between about 50 andabout 100, between about 100 and about 500, or between about 100 andabout 300.

In various embodiments, the transparent conductive electrode 130 mayfurther include a substrate 132 on which the nanowire network 120 a maybe provided. The substrate 132 may include a flexible substrate. Thesubstrate 132 may include an organic substrate. In the context ofvarious embodiments, the substrate 132 may include at least one ofpolyimide (PI), polycarbonate (PC), polyethersulfone (PES),polyethyleneterephthalate (PET), polyethylenenaphthalate (PEN) orpolyarylate (PAR).

In the context of various embodiments, the plurality of conductivenanowires 122 a may include a metal and/or the plurality of conductivenanoparticles 124 a may include a metal. The metal may be selected fromthe group consisting of copper (Cu), silver (Ag) and gold (Au). Invarious embodiments, the plurality of conductive nanowires 122 a and theplurality of conductive nanoparticles 124 a may include or may be madeof the same metal.

In the context of various embodiments, the plurality of conductivenanowires 122 a and the plurality of conductive nanoparticles 124 a mayinclude or consist essentially of copper (Cu).

In various embodiments, each conductive nanowire 122 a of the pluralityof conductive nanowires 122 a may include a surfactant on a surface ofthe conductive nanowire 122 a. The surfactant may prevent or minimizeagglomeration of the plurality of conductive nanowires 122 a. Thesurfactant may include a thiol or an amine.

In the context of various embodiments, the transparent conductiveelectrode 130 may be used in a flexible touchscreen or display.

In the context of various embodiments, the terms “fuse” and “fusing” maymean sintering, or joining together as an (single) entity. This may meanthat there may not be clear or obvious boundary observable between twomaterials (or structures) when the two materials are fused to eachother. Further, the two materials fused to each other may not beseparate or distinct.

It should be appreciated that descriptions in the context of thenanowire network 120 and the transparent conductive electrode 130 may becorrespondingly applicable to each other, and may also becorrespondingly applicable in relation to the method for interconnectingnanowires, and vice versa.

Various embodiments may provide a copper nanowires-nanoparticles mixturefor transparent conducting electrodes.

Various embodiments may provide a composition of copper nanowires (CuNWs) and copper nanoparticles (Cu NPs) that may allow low temperatureprocessing (for example, <250° C.) to be compatible with flexibleorganic substrates, with the required electrical conductivity andoptical transmittance as a transparent electrode. Various embodimentsmay include or provide one or more of the following:

-   -   (i) To enable low temperature process, very small (for example,        <10 nm) Cu nanoparticles (NPs) which may fuse between about        100° C. and about 250° C. may be used as the joining material        between Cu nanowires to provide the electrical conductivity.        Therefore, the process temperature may be decreased to less than        about 250° C. Moreover, the nanoparticles formed joint lowers        the contact resistance in comparison to a direct        nanowire-nanowire joint. PCT/US2010/039069 describes a method        for forming small Cu nanoparticles and the fabricated small        copper nanoparticles that may be used in various embodiments        described herein, the entire disclosure of which is incorporated        herein by reference. Nevertheless, copper NPs which are produced        by other methods may also be used as the joining material.        However, the size and size distribution of the NPs employed in        various embodiments may change the required process temperature        and temperature profile for fusing the nanostructures or        nanowires. From preliminary experiments, the inventors found        that if NPs with 40-100 nm diameter are used, the process        temperature required is in the range of 300-350° C. The blended        composition of nanowires and nanoparticles as the fusing        material of various embodiments may be employed as a transparent        electrode.    -   (ii) Long (for example, up to 50 μm) Cu nanowires may be used in        various embodiments, which may reduce the number of        nanowire-nanowire contact and thus may increase the electrical        conductivity (for example, for large area transparent conductive        electrodes), but also may lead to a ductile mechanical property        which may enable the flexibility required.    -   (iii) Mixing NWs and NPs in a low viscosity liquid (e.g.,        ethanol, isopropyl alcohol (IPA)) may encourage or cause the NWs        to disperse uniformly, and may aid the NPs to coalesce at the        junctions of contacting NWs. For this, the solvent should be        fluid enough for the NPs to move in the dispersion. This        mechanism may be explained thermodynamically that the NPs tend        to reduce their high surface energy by fusing and increasing        their contact area at joints, and the NPs have the required        mobility to move. From another perspective, if the NPs are        well-dispersed, for example, across a substrate, during the        drying and washing process, more NPs may be placed at the        junction of NWs and fused during the annealing process.    -   (iv) In order to achieve better dispersion of the mixture of NWs        and NPs, one or more chemical treatments before mixing the NWs        and NPs may be effective. As non-limiting examples, different        types of thiol may be used as a surfactant to attach the diverse        length of carbon chains on the surface of the NWs (e.g., Cu        NWs). However, it should be appreciated that other kinds of        chemicals may also be used to substitute for thiol which may        make the NWs more stable and prevent or at least minimize        agglomeration for better dispersion.

In various embodiments, copper (Cu) nanoparticles and copper (Cu)nanowires may be mixed with an appropriate (weight) ratio to address theproblems described above. Cu nanowires are used as the main conductiveand transparent material with Cu nanoparticles as a joining materialbetween the nanowires.

FIG. 2A shows, as cross-sectional views, various processing stages of amethod 240 of manufacturing copper (Cu) nanowires (NWs), according tovarious embodiments, illustrating a procedure of manufacturing of Cu NWsusing anodized aluminum oxide (AAO). In general, gold/copper (Au/Cu)layers may be deposited on the AAO as a seed layer for electroplating.Cu may subsequently be electroplated into the holes in the AAO. Then,the seed layer and the AAO may be etched out by an etchant as shown inFIG. 2A. Finally, the Cu NWs may be detached from the substrate.Therefore, Cu nanowires may be produced through electroplating usinganodic aluminum oxide (AAO) as a template. As a result, uniform lengthof nanowires may be obtained. While it is described herein that theelectroplating method with AAO as a template may be used to obtain theCu nanowires, it should be appreciated that a chemical synthesis or aphysical deposition process may be used instead.

As a non-limiting example, referring to FIG. 2A, an anodic aluminumoxide (AAO) template 242 may first be provided or prepared. The AAOtemplate 242 may have a plurality of holes (or pores or channels) 243.The AAO template 242 may be sputtered with Au and Cu, where a 0.2 nmthick gold layer 244 and a 1 μm thick copper layer 246 may be obtained.

The AAO template 242 may then be attached onto a cathode (not shown) and50 μm long Cu nanowires 222 may be electrochemical synthesized in theholes 243 of the AAO template 242.

The AAO template 242 may be attached to a thermal tape 248. Thesputtered Cu layer 246 and Au layer 244 may be etched away by chemicaletching processes. The AAO template 242 may be etched using a sodiumhydroxide (NaOH) solution. As a result, free Cu nanowires 222 may beobtained.

The Cu nanowires 222 may be washed with ethanol, followed by isopropylalcohol (IPA).

In various embodiments, optionally or if necessary, the nanowires 222may be coated with thiol or amine group. In this way, thiol or amine asa surfactant may be coated on the nanowires 222.

Subsequently, Cu nanoparticles may be added to the nanowires 222 formixing. The mixture may be added into a solution (or solvent) (e.g.,isopropyl alcohol (IPA)) for dispersion and may be coated by spincoating, for example, onto a substrate. As a non-limiting example,referring to FIG. 2B illustrating a method 250 of mixing of coppernanoparticles (Cu NPs) and copper nanowires (Cu NWs), Cu NPs 224 (e.g.,having diameters of about 5-20 nm) are added to Cu NWs 222 (e.g., havinglengths of about 20-50 m) and the two materials may be mixed in anultra-sonicator and washed with an alcohol base solution (e.g.,isopropyl alcohol (IPA)) 251. Then, the solution (e.g., in the form ofdroplet 252) containing the mixture of Cu NWs 222 and Cu NPs 224, may bedeposited, for example, from a dispenser 254, onto a substrate 232, andthen dispersed on the substrate 232 by diverse methods, such as spincoating, mayor bar coating, roll to roll coating, spray coating, etc.

FIGS. 2C and 2D show examples of some steps of the processing method ofvarious embodiments, using copper nanowires and copper nanoparticles asexamples.

Referring to FIG. 2C, after forming copper nanowires using an AAOtemplate 242, the AAO template 242 may be removed or etched by immersingthe AAO template 242 with the copper nanowires in a solution 255 ofsodium hydroxide (NaOH) (0.5 M concentration) inside a container 256.The solution 255 with the free copper nanowires may be centrufuged andthen, the solution 255 may be drained off, leaving behind coppernanowires 222 inside the container 256. Then, an appropriate amount ofthe copper nanowires 222 may be weighed and copper nanoparticles 224 maythen be added to the copper nanowires 222 until the desired weight ratioof the copper nanowires 222 to the copper nanoparticles 224 is obtained,for example about 20:1. The mixture of the copper nanowires 222 and thecopper nanoparticles 224 may be washed in an alcohol based solution(e.g., ethanol, IPA) 251. The copper nanowires 222 and the coppernanoparticles 224 may be subsequently mixed in the solvent (alcoholbased solution 251) using an ultrasonicator, where the weight ratio (orweight percentage) of solvent: mixture of copper nanowires 222 andcopper nanoparticles 224 is 10:1.

Referring to FIG. 2D, copper (Cu) nanowires 222 may be dispersed in analcohol based solvent 257 in a container 258. Copper (Cu) nanoparticles224 may be added into the alcohol based solvent 257 and the solution 259containing the Cu nanowires 222, the Cu nanoparticles 224 and thealcohol based solvent 257 may be mixed by an ultrasonicator. Then, thesolution 259 or part thereof, after mixing, may be deposited ordispensed (e.g., in the form of droplet 252) from a dispenser 254 onto asubstrate 232, and subsequently dispersed or coated as a layer 252 a onthe substrate 232. A heating process may then be carried out at atemperature between about 120° C. and about 250° C., for example, frombelow the substrate 232. As a result of the heating process, the alcoholbased solvent 257 may be evaporated and the Cu nanoparticles 224 mayfuse to each other and to the Cu nanowires 222.

It should be appreciated that the methods or steps described in thecontext of FIGS. 2A-2D respectively may be applicable also to othermethods or steps of FIGS. 2A-2D, or may be combined in any manner.

FIG. 3A show a scanning electron microscope (SEM) image of coppernanowires 322 after manufacturing, for example, based on the method 240described in the context of FIG. 2A. As shown in FIG. 3A, extremely longCu NWs (>50 μm) 322 may be obtained. FIG. 3B shows a scanning electronmicroscope (SEM) image of copper nanowires 322 a after dispersion on asubstrate (not clearly shown).

In various embodiments, after being dispersed onto a substrate, thecopper nanowires and the nanoparticles may be subjected to a heatingprocess. The heating process may assist or encourage joining of the Cunanowires and the Cu nanoparticles, where the Cu nanoparticles may befused to the Cu nanowires to interconnect the Cu nanowires via the Cunanoparticles. FIG. 4 shows a schematic diagram 460 illustrating themechanism in which copper (Cu) nanoparticles 424 fuse at the junctions426 of copper (Cu) nanowires 422, so as to achieve the requiredelectrical conductivity at low temperatures.

In various embodiments, Cu nanowires with a diameter of about 20-200 nmand a length of about 20-50 μm (aspect ratio: 100-500) and Cunanoparticles with a diameter of about 5-20 nm are used. The weightratio of Cu nanowires to Cu nanoparticles is from about 10:1 to about20:1.

Examining a range of sizes, lengths, weight ratios of Cu nanowires andCu nanoparticles, an optimal composition may be determined for therequired electrical conductivity and optical transmittance (see, forexample, FIGS. 5A, 5B, 5C and 6 to be described below).

FIG. 5A shows scanning electron microscope (SEM) images of coppernanowires (Cu NWs) 522 a, illustrating samples made by only Cu nanowires522 a.

FIG. 5B shows scanning electron microscope (SEM) images ofmicrostructures of mixed copper (Cu) nanowires 522 b and copper (Cu)nanoparticles 524 b which are annealed at about 200° C., according tovarious embodiments. The weight ratio of the nanowires 522 b to thenanoparticles 524 b is 10:0.6. As may be observed in FIG. 5B, the Cunanoparticles 524 b are fused to the Cu nanowires 522 b at junctions 526b between the Cu nanowires 522 b. In this way, the Cu nanowires 522 bmay be interconnected to each other via Cu nanoparticles 524 b at thejunctions 526 b.

FIG. 5C shows scanning electron microscope (SEM) images ofmicrostructures of mixed copper (Cu) nanowires 522 c and copper (Cu)nanoparticles 524 c which are annealed at about 200° C., according tovarious embodiments. For obtaining the SEM images shown in FIG. 5C,three drops of the mixture of copper (Cu) nanowires 522 c and copper(Cu) nanoparticles 524 c were provided on a substrate for coating thesubstrate. The weight ratio of the nanowires 522 c to the nanoparticles524 c is 8:1.5. As may be observed in FIG. 5C, the Cu nanoparticles 524c are fused to the Cu nanowires 522 c at junctions 526 c between the Cunanowires 522 c. In this way, the Cu nanowires 522 c may beinterconnected to each other via Cu nanoparticles 524 c at the junctions526 c. The SEM image 590 shows a cross sectional view of the mixedcopper (Cu) nanowires 522 c and copper (Cu) nanoparticles 524 c usingFIB (Focused Ion Beam).

FIG. 6 shows a plot 670 of sheet resistance (unit: ohm per square, Ω/□)for various copper (Cu) nanostructures mixtures on polyimide substrates.Plot 670 shows result 672 for copper nanowires (NWs), result 674 forcopper nanowires with Cu nanoparticles fused to the nanowires where onedrop of the mixture of copper (Cu) nanowires and copper (Cu)nanoparticles was provided on a polyimide substrate (NWs+NPs X1, where“X1” represents one drop) and result 676 for copper nanowires with Cunanoparticles fused to the nanowires where three drops of the mixture ofcopper (Cu) nanowires and copper (Cu) nanoparticles were provided on apolyimide substrate (NWs+NPs X3, where “X3” represents three drops). Asshown in FIG. 6, the sheet resistance may decrease with the addition ofnanoparticles to the nanowires and may further decrease as the amount ofnanoparticles is increased.

FIG. 7A shows a plot 770 of sheet resistance of fused copper (Cu)nanowires-nanoparticles on different substrates. The composition rate ofthe mixture of Cu NWs to the Cu NPs is about 20(NW):1(NP). Plot 770shows the sheet resistances of Cu nanowires and Cu nanoparticlescomposite on different substrates such as result 772 for quartz, result774 for polyimide, result 776 for polyimide (same substratecorresponding to result 774) and after bending of the polyimide sample(indicated as “1st” to refer to one (first) time of bending), result 778for polyimide (same substrate corresponding to results 774, 776) andafter bending of the polyimide sample (indicated as “2nd” to refer tosecond time of bending) and result 780 for polyethyleneterephthalate(PET). In various embodiments, electrical conductivity is maintainedeven after preliminary flexing of the polyimide substrate (e.g., 782),as may be observable from results 776, 778.

FIG. 7B shows a plot 786 of optical transmittance of a transparentcopper nanowires (Cu NWs) with copper nanoparticles (Cu NPs) electrode,measured with an ultraviolet-visible (UV-vis) spectrometer, for a mix ofCu nanowires-nanoparticles having a composition rate of Cu NWs to Cu NPsof about 20(NW):1(NP). An optical transmittance of about 40% may beobtained.

In various embodiments, some polymers (such as polymethylmethacrylate(PMMA), polystyrene, etc.) may aid in the dispersion of thenanostructures (by improving the dispersion properties) and may lead toa smooth surface as shown by the SEM images in FIGS. 8A, 8B and 8C. Theuse of PMMA as a matrix may provide advanced or improved coatinguniformity, and higher adhesion on a substrate (e.g., glass substrate).Therefore, in various embodiments, Cu NWs and Cu NPs may be mixed in asolution containing a polymer such as PMMA.

FIG. 8A shows a schematic diagram illustrating a solution ofpolymethylmethacrylate (PMMA) 890 a with copper (Cu) nanowires 822 adispensed onto a substrate 832, according to various embodiments. No CuNPs were included.

FIG. 8B shows scanning electron microscope (SEM) images illustrating theeffects of different concentrations (weight %) of polymethylmethacrylate(PMMA) 890 b for dispersion of nanowires 822 b. No Cu NPs were included.

FIG. 8C shows scanning electron microscope (SEM) images of copper (Cu)nanowires in polymethylmethacrylate (PMMA). No Cu NPs were included. 4wt % PMMA (molecular weight of 996K) in chloroform solvent was used andthe PMMA solution 890 c containing nanowires 822 c dispersed therein wasspin-coated at about 2000 rpm onto a glass substrate, and then annealedat about 350° C. The processing further included treatment of thenanowires 822 c with a thiol to prevent or minimise agglomeration of thenanowires 822 c, for example, treatment with octanethiol where thenanowires 822 c may be stirred in the thiol at about 500 rpm at about80° C. Generally, treatment with a surfactant such as a thiol or aminemay be carried out before mixing with nanoparticles. As a non-limitingexample, the entire process flow may be as follows: Nanowire (rawmaterial)→Treatment with thiol or amine→Adding nanoparticles→Mixing withmatrix (e.g., PMMA)→Dispersing(coating)→Heating (annealing).

As non-limiting examples, different molecular weights (MW) of PMMA(e.g., 112 k, 120 k, 996K) and/or concentrations (e.g., 0.5%-10%, e.g.,0.1-0.4 wt %), and the concentration of nanowires in PMMA or solvent(e.g., 5%-20%) may be used. Dispersion property may be diverse ordifferent according to the concentration and/or MW of PMMA. Forembodiments employing PMMA as a matrix, the solvent used for PMMA mayinclude dimethylformamide (DMF) (boiling point ˜153° C.), chloroform(boiling point ˜61° C.), or toluene (boiling point ˜110° C.).

In various embodiments, conductive polymers such aspoly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) mayalso be employed as the matrix to improve the electrical properties asoccasion demands.

FIG. 9A shows schematic diagrams illustrating the use of an anodicaluminium oxide 932 as a substrate in the method of various embodiments.A solution containing PMMA (as matrix) 990 and copper (Cu) nanowires 922may be provided on the anodic aluminium oxide 932. Over time, the PMMA990 may flow through the holes (or channels) 933 of the anodic aluminiumoxide 932, out of the anodic aluminium oxide 932 on the side of theanodic aluminium oxide 932 opposite to that having the Cu nanowires 922.As a result, only minimal or residual PMMA 990 may remain with the Cunanowires 922. A substrate (e.g., glass) 992 having an adhesive (e.g.,epoxy glue) 994 may be used to transfer the Cu nanowires 922 remainingon the anodic aluminium oxide 932 onto the substrate 992. Therefore, invarious embodiments, the anodic aluminium oxide 932 may act as anintermediate substrate for transferring the Cu nanowires 922 to the(final) substrate 992. While FIG. 9A does not show the addition of Cunanoparticles, it should be appreciated that Cu nanoparticles may beadded to the nanowires 922 before mixing with the matrix (PMMA 990).

In various embodiments, when anodic aluminium oxide 932 is used as asubstrate, the annealing temperature employed may go higher than about300° C. because the melting temperature of AAO is about 2,000° C. WhilePMMA is used to improve the dispersion of the nanowires, the presence ofPMMA may adversely affect the performance of a transparent electrode andso the preferred way is to remove/reduce the PMMA, for example, byheating. The annealing temperature depends on the molecular weight andconcentration of the PMMA, however, a temperature of about 400° C. maybe an optimum treatment temperature. The annealing process may becarried out after the second step illustrated in FIG. 9A, meaning afterdispersion of the nanowires 922 onto the anodic aluminium oxide 932 andprior to transferring to the substrate 992. In other words, the processflow may include dispersion of NWs onto the AAO→annealing →transferring.

FIG. 9B shows photographs illustrating the transfer of nanowires (e.g.,Cu nanowires) 922 a coated on an anodic aluminium oxide 932 a from theanodic aluminium oxide 932 a to another substrate, e.g., PET 992 a withadhesive, according to various embodiments. As shown in FIG. 9B, theanodic aluminium oxide 932 a with coated nanowires 922 a (with PMMA) mayfirst be provided affixed to a glass substrate 933 a having an adhesivetape 934 a. The nanowires 922 a (with PMMA 990 a) and the anodicaluminium oxide 932 a may then be transferred to the PET 992 a. Theanodic aluminium oxide 932 a may then be transferred back to the glasssubstrate 933 a with the adhesive tape 934 a, leaving behind thenanowires 922 a (possibly with some residual PMMA 990 a) on the PET 992a. As may be observed in FIG. 9B, the PET 992 a having the nanowires 922a may be optically transparent.

FIG. 9C shows a scanning electron microscope (SEM) image ofwell-dispersed nanowires 922 c on an anodic aluminium oxide (AAO)substrate 932 c. While present, nanoparticles are not clearly shown inthe SEM image. The weight ratio of the nanowires 922 c to thenanoparticles was about 20:1, the solvent used was IPA and the annealingtemperature was about 300° C.

Some functional groups (e.g., thiol and amine) may act as a surfactantto enhance the dispersion property of the mixture. The effect of addingsome thiol or amine into the mixture of NPs and NWs before dispersionmay be determined, because these molecules attach on the surface of theNWs (see FIG. 10 illustrating thiol molecules 1095 attached on thesurface of nanowires (NWs) (e.g., Cu NWs)) 1022, where these molecules1095 work like arms causing the NWs 1022 to repel each other and preventagglomeration.

In various embodiments, PMMA may be the dispersion matrix and thiol maybe the surfactant. PMMA may be used as the dispersion matrix to hold theCu nanowires uniformly across the substrate to prevent or at leastminimise agglomeration of the Cu nanowires and improves the conductivityof the thin films. The PMMA may be subsequently removed.

As described above, various embodiments may include or provide one ormore of the following:

-   -   (i) A much lower process temperature (about 100-250° C.) to form        the conductive electrode due to the fusion temperature of small        copper (Cu) nanoparticles used. This is a major improvement over        existing technology which has a higher processing temperature        (300-500° C.). The lower temperature (<250° C.) for various        embodiments may enable compatibility of the process with        flexible organic substrates.    -   (ii) Long, uniform and high aspect ratio (about 50-250) Cu        nanowires may be used in the composition. This may improve        flexibility while maintaining the electrical conductivity and        the optical transmittance required.    -   (iii) Cu may be used as both the conductive and joining        materials. For example, by using only Cu as an electrode        material, aggressive inter-diffusion and reaction between        heterogeneous materials may be avoided.

Various embodiments may be related to or focused on the touchscreen anddisplay applications. In order to apply to flexible touchscreens ordisplays, the process temperature has to be lower than 250° C., whilethe electrical and optical properties have to be maintained afterrepeated bending or straining. The global market for transparentconductive coatings is expected to grow to nearly $7.1 billion by 2018,and for flexible displays, reached $39.1 million in 2012.

While the invention has been particularly shown and described withreference to specific embodiments, it should be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims. The scope of the invention is thusindicated by the appended claims and all changes which come within themeaning and range of equivalency of the claims are therefore intended tobe embraced.

The invention claimed is:
 1. A method of interconnecting nanowirescomprising: mixing together a plurality of nanowires and a plurality ofnanoparticles to form a mixture; depositing the mixture upon asubstrate; and once deposited upon the substrate, fusing the pluralityof nanoparticles to the plurality of nanowires at junctions wherenanowires come together to interconnect the plurality of nanowires toeach other, the nanoparticles being further fused to each other; whereineach nanowire of the plurality of nanowires comprises a surfactant on asurface of the nanowire, the surfactant being provided on the surfacebefore mixing the plurality of nanowires with the plurality ofnanoparticles.
 2. The method as claimed in claim 1, wherein fusing theplurality of nanoparticles to the plurality of nanowires comprisessubjecting the plurality of nanowires and the plurality of nanoparticlesto a heating process, and wherein the heating process is carried out ata predetermined temperature of 250° C. or less.
 3. The method as claimedin claim 1, wherein a weight ratio of the plurality of nanowires to theplurality of nanoparticles is between 5:1 and 20:1.
 4. The method asclaimed in claim 1, wherein the plurality of nanowires and the pluralityof nanoparticles are mixed in a solvent, and the solvent is removedprior to fusing the plurality of nanoparticles to the plurality ofnanowires.
 5. The method as claimed in claim 1, wherein the substratecomprises at least one of polyimide, polycarbonate, polyethersulfone,polyethyleneterephthalate, polyethylenenaphthalate or polyarylate. 6.The method as claimed in claim 1, wherein the plurality of nanoparticlescomprise a metal.
 7. The method as claimed in claim 1, wherein theplurality of nanowires and the plurality of nanoparticles comprisecopper.
 8. The method as claimed in claim 1, wherein the surfactantcomprises a thiol or an amine.
 9. The method as claimed in claim 1,wherein the plurality of nanoparticles comprise copper nanoparticles.10. A transparent conductive electrode comprising: a nanowire network,the nanowire network comprising a plurality of conductive nanowiresinterconnected to each other via a plurality of conductive nanoparticlesfused to the plurality of conductive nanowires at junctions whereconductive nanowires come together, the conductive nanoparticles beingfurther fused to each other.
 11. The method as claimed in claim 1,wherein each nanoparticle of the plurality of nanoparticles has a sizebetween about 5 nm and about 20 nm.
 12. The transparent conductiveelectrode as claimed in claim 10, wherein the plurality of conductivenanoparticles comprise copper nanoparticles.
 13. The transparentconductive electrode as claimed in claim 10, further comprising asubstrate on which the nanowire network is provided, wherein thesubstrate comprises at least one of polyimide, polycarbonate,polyethersulfone, polyethyleneterephthalate, polyethylenenaphthalate orpolyarylate.
 14. The transparent conductive electrode as claimed inclaim 10, wherein the plurality of conductive nanoparticles comprise ametal.
 15. The transparent conductive electrode as claimed in claim 10,wherein the plurality of conductive nanowires and the plurality ofconductive nanoparticles comprise copper.